It is required recently to improve fuel economy and reduce environmentally harmful substances contained in combustion emissions of an internal combustion engine of a vehicle. For this purpose, an ignition device is required to provide superior ignition performance even in case of lean fuel mixture or turbo-charged fuel mixture.
A conventional power supply device for a high frequency ignition system (for example, JP 2010-522841A corresponding to US 2010/0116257 A1) includes a power supply circuit for supplying a plasma generation resonance circuit with a power source voltage at a frequency determined by a control signal outputted from a power supply circuit control device. The control device includes an interface for receiving a determination request for an optimal control frequency, an interface for receiving a detection signal indicating a measured voltage of a capacitor terminal of the power supply circuit, and a module for determining an optimal control frequency. This module supplies, when requested, the power supply circuit with series of different control frequencies for continuous ignition instruction. The module further determines the optimal control frequency based on the received detection signal.
According to the conventional high frequency ignition system, a high voltage is applied to each terminal of a resonator of a high frequency plug and a coil to generate sparks between electrodes of the plug and the coil. The high frequency resonator of the plug and the coil is driven at its resonance frequency only when a difference between the detection voltages measured at the capacitor terminal of the power supply circuit at time of starting ignition and time of terminating ignition is maximum. By using an electrically measured value of the voltage of the capacitor terminal, the optimal control frequency, which generally corresponds to the resonance frequency of the plasma generation resonator, is determined and stored. By using the stored optimal control frequency, energy supplied to the resonator formed of the plug and the coil is maximized.
According to the conventional high frequency plasma ignition device, when a pressure in a combustion chamber rises, a withstand voltage rises and a required voltage for starting the discharge increases. However, when the pressure in the combustion chamber rises due to abnormal combustion such as detonation (knocking), the pressure rise occurs after an insulating body in a discharge space has been broken by the high voltage supplied from the high voltage power source and the discharge has started. For this reason, the density of mixture present in the discharge space is increased and hence the discharge space allows current to flow readily therein. The amount of current flowing when the high frequency energy is inputted becomes greater than that flowing in a normal combustion condition. Thus, high frequency plasma of extremely large energy generates and causes remarkable wear in the discharge electrodes.
In the conventional high frequency plasma ignition device, the discharge part is not insulated from other control circuits. The high frequency noise generated at the discharge time affects control circuits such as an ECU and causes erroneous operation of the ECU and the like. Since not only the drive frequency is as high as more than 1 MHz but also the electric power supplied instantaneously is large, it is difficult to separate electrically by a normal transformer. It is thus necessary to effectively counter high frequency noises.